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我已经改变了问题的标题，以反映我的问题，但也回答了如何轻松实现这一目标。
我正在努力使第二种方法返回Task<TResult>而不是Task，就像第一种方法一样，但是由于尝试修复它而导致一连串的错误。
return await body(partition.Current);
return null Task.Run更改为Task.Run<TResult>但未成功。我该如何解决？
第一种方法来自http://blogs.msdn.com/b/pfxteam/archive/2012/03/05/10278165.aspx，第二种方法是我正在尝试创建的重载。
public static class Extensions
{
public static Task ForEachAsync<T>(this IEnumerable<T> source, int dop, Func<T, Task> body)
{
return Task.WhenAll(
from partition in Partitioner.Create(source).GetPartitions(dop)
select Task.Run(async delegate
{
using (partition)
while (partition.MoveNext())
await body(partition.Current);
}));
}
public static Task ForEachAsync<T, TResult>(this IEnumerable<T> source, int dop, Func<T, Task<TResult>> body)
{
return Task.WhenAll(
from partition in Partitioner.Create(source).GetPartitions(dop)
select Task.Run(async delegate
{
using (partition)
while (partition.MoveNext())
await body(partition.Current);
}));
}
}
用法示例：
使用这种方法，我想并行和异步地下载多个文件：
private async void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
Artist artist = await GetArtist();
IEnumerable<string> enumerable = artist.Reviews.Select(s => s.ImageUrl);
string[] downloadFile = await DownloadFiles(enumerable);
}
public static async Task<string[]> DownloadFiles(IEnumerable<string> enumerable)
{
if (enumerable == null) throw new ArgumentNullException("enumerable");
await enumerable.ForEachAsync(5, s => DownloadFile(s));
// Incomplete, the above statement is void and can't be returned
}
public static async Task<string> DownloadFile(string address)
{
/* Download a file from specified address,
* return destination file name on success or null on failure */
if (address == null)
{
return null;
}
Uri result;
if (!Uri.TryCreate(address, UriKind.Absolute, out result))
{
Debug.WriteLine(string.Format("Couldn't create URI from specified address: {0}", address));
return null;
}
try
{
using (var client = new WebClient())
{
string fileName = Path.GetTempFileName();
await client.DownloadFileTaskAsync(address, fileName);
Debug.WriteLine(string.Format("Downloaded file saved to: {0} ({1})", fileName, address));
return fileName;
}
}
catch (WebException webException)
{
Debug.WriteLine(string.Format("Couldn't download file from specified address: {0}", webException.Message));
return null;
}
}
答案 0 :(得分：27)
我解决了问题并将其发布在此处，可能有助于任何人遇到同样的问题。
我最初的需求是一个小帮手，可以快速下载图像，但如果服务器没有快速响应，只需断开连接，所有这些并行和异步。
这个帮助器会返回一个元组，其中包含远程路径，本地路径和异常（如果发生）;非常有用，因为知道为什么错误的下载有故障总是好的。我想我没有忘记下载的任何情况，但欢迎您发表评论。
您可以使用DownloadFileTaskAsync本身或使用ForEachAsync帮助程序进行并行和异步下载。
代码以及如何使用它的示例：
private async void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
IEnumerable<string> enumerable = your urls here;
var results = new List<Tuple<string, string, Exception>>();
await enumerable.ForEachAsync(s => DownloadFileTaskAsync(s, null, 1000), (url, t) => results.Add(t));
}
/// <summary>
/// Downloads a file from a specified Internet address.
/// </summary>
/// <param name="remotePath">Internet address of the file to download.</param>
/// <param name="localPath">
/// Local file name where to store the content of the download, if null a temporary file name will
/// be generated.
/// </param>
/// <param name="timeOut">Duration in miliseconds before cancelling the operation.</param>
/// <returns>A tuple containing the remote path, the local path and an exception if one occurred.</returns>
private static async Task<Tuple<string, string, Exception>> DownloadFileTaskAsync(string remotePath,
string localPath = null, int timeOut = 3000)
{
try
{
if (remotePath == null)
{
Debug.WriteLine("DownloadFileTaskAsync (null remote path): skipping");
throw new ArgumentNullException("remotePath");
}
if (localPath == null)
{
Debug.WriteLine(
string.Format(
"DownloadFileTaskAsync (null local path): generating a temporary file name for {0}",
remotePath));
localPath = Path.GetTempFileName();
}
using (var client = new WebClient())
{
TimerCallback timerCallback = c =>
{
var webClient = (WebClient) c;
if (!webClient.IsBusy) return;
webClient.CancelAsync();
Debug.WriteLine(string.Format("DownloadFileTaskAsync (time out due): {0}", remotePath));
};
using (var timer = new Timer(timerCallback, client, timeOut, Timeout.Infinite))
{
await client.DownloadFileTaskAsync(remotePath, localPath);
}
Debug.WriteLine(string.Format("DownloadFileTaskAsync (downloaded): {0}", remotePath));
return new Tuple<string, string, Exception>(remotePath, localPath, null);
}
}
catch (Exception ex)
{
return new Tuple<string, string, Exception>(remotePath, null, ex);
}
}
public static class Extensions
{
public static Task ForEachAsync<TSource, TResult>(
this IEnumerable<TSource> source,
Func<TSource, Task<TResult>> taskSelector, Action<TSource, TResult> resultProcessor)
{
var oneAtATime = new SemaphoreSlim(5, 10);
return Task.WhenAll(
from item in source
select ProcessAsync(item, taskSelector, resultProcessor, oneAtATime));
}
private static async Task ProcessAsync<TSource, TResult>(
TSource item,
Func<TSource, Task<TResult>> taskSelector, Action<TSource, TResult> resultProcessor,
SemaphoreSlim oneAtATime)
{
TResult result = await taskSelector(item);
await oneAtATime.WaitAsync();
try
{
resultProcessor(item, result);
}
finally
{
oneAtATime.Release();
}
}
}
我没有更改ForEachAsync的签名来选择并行级别，我会让你按照自己的意愿调整它。
输出示例：
DownloadFileTaskAsync (null local path): generating a temporary file name for http://cache.thephoenix.com/secure/uploadedImages/The_Phoenix/Music/CD_Review/main_OTR_Britney480.jpg
DownloadFileTaskAsync (null local path): generating a temporary file name for http://ssimg.soundspike.com/artists/britneyspears_femmefatale_cd.jpg
DownloadFileTaskAsync (null local path): generating a temporary file name for http://a323.yahoofs.com/ymg/albumreviewsuk__1/albumreviewsuk-526650850-1301400550.jpg?ymm_1xEDE5bu0tMi
DownloadFileTaskAsync (null remote path): skipping
DownloadFileTaskAsync (time out due): http://hangout.altsounds.com/geek/gars/images/3/9/8/5/2375.jpg
DownloadFileTaskAsync (time out due): http://www.beat.com.au/sites/default/files/imagecache/630_315sr/images/article/header/2011/april/britney-spears-femme-fatale.jpg
DownloadFileTaskAsync (time out due): http://cache.thephoenix.com/secure/uploadedImages/The_Phoenix/Music/CD_Review/main_OTR_Britney480.jpg
DownloadFileTaskAsync (downloaded): http://newblog.thecmuwebsite.com/wp-content/uploads/2009/12/britneyspears1.jpg
DownloadFileTaskAsync (downloaded): http://newblog.thecmuwebsite.com/wp-content/uploads/2009/12/britneyspears1.jpg
DownloadFileTaskAsync (downloaded): http://static.guim.co.uk/sys-images/Music/Pix/site_furniture/2011/3/22/1300816812640/Femme-Fatale.jpg
DownloadFileTaskAsync (downloaded): http://www.sputnikmusic.com/images/albums/72328.jpg
过去需要1分钟的时间现在只需10秒钟就可以获得相同的结果：）
非常感谢这两篇文章的作者：
http://blogs.msdn.com/b/pfxteam/archive/2012/03/05/10278165.aspx
http://blogs.msdn.com/b/pfxteam/archive/2012/03/04/10277325.aspx